1. Field of the Invention
This invention relates to a fluorescent display device wherein a display section provided with phosphor layers and a circuit section provided with a semiconductor chip are arranged on the same substrate in a vacuum envelope, and more particularly to a fluorescent display device which is improved in a structure of the circuit section and a die bond layer to improve reliability in its operation and a method for manufacturing the same.
2. Description of the Prior Art
The prior art will be described with reference to FIGS. 3 and 4.
A conventional fluorescent display device of the type that a display section 1 provided with phosphor layers and a circuit section 3 provided with an IC chip are arranged on the same substrate 4 in a vacuum envelope (not shown) is shown in FIG. 3. It is generally called a chip-in-glass type fluorescent display device. In the device, the circuit section 3 provided on the substrate 4 is constructed in such a manner as shown in FIG. 4A and 4B. More particularly, as shown in FIG. 4B, a die bonding pad 5 made of a metal material such as Al or the like is provided on an upper surface of the substrate 4 and entirely covered with a die bond layer 6 formed of a conductive adhesive. On the die bonding pad 5 is fixed an IC chip 2 through the die bond layer 6. From the die bonding pad 5 is led out an earth wire 5a to keep a silicon substrate of the IC chip 2 at ground potential. Around the die bonding pad 5 are a plurality of bonding pads 7 so as not to be contacted with the die bond layer 6. The bonding pads 7 each are connected through a wire conductor 8 formed of Al, Ag or the like to an input-output terminal of the IC chip 2. From a side of each of the bonding pads 7 opposite to its side facing the die bonding pad 5 is led out a wiring 7a onto the substrate 4. The wirings 7a each are guided so as to avoid a region between the die bonding pad 5 and the bonding pads 7 for connection to the display section 1, lead terminals and the like. Manufacturing of the chip-in-glass type fluorescent display device requires to heat the envelope to 400.degree.-500.degree. C. during hermetic sealing of the envelope. The heating causes the IC chip 2 arranged in the envelope to be subjected to a high temperature. Accordingly, it is necessary to determine a thickness of the die bond layer 6 for bonding the IC chip 2 and the substrate 4 together therethrough in view of not only its bonding strength but thermal stress occurring in the die bond layer 6 due to the heating. This leads to an increase in the thickness to a degree sufficient to cause the die bond layer 6 to absorb the thermal stress. Concurrently, it is required to render the thickness substantially uniform.
In general, the circuit section 3 of the chip-in-glass type fluorescent display device is made by putting the die bond layer 6 of a paste-like state on the die bonding pad 5, positioning the IC chip 2 on the die bond layer 6 while pressing it, and then fixing the IC chip on the die bond layer 6 by baking. Unfortunately, during formation of the circuit section 3, the pasty die bond layer 6 not only outwardly expands by gravity but is forced out from between the IC chip 2 and the die bonding pad 5 because the IC chip 2 is downwardly forced against the die bond layer 6. Thus, the prior art fails to form the die bond layer 6 of a large and uniform thickness.
A polyimide paste has been conventionally used as a paste for such a die bond layer as described above, as disclosed in Japanese Utility Model Application Laying-Open Publication No. 24946/1986. However, the polyimide paste starts thermal decomposition at a temperature of about 430.degree. C., accordingly, use of the polyimide paste for a fluorescent display device causes decomposition of the paste to produce various kinds of destructive gas because it is exposed to a high temperature of 400.degree. to 500.degree. C. for hermetic sealing of a vacuum envelope, resulting in deteriorating characteristics of the fluorescent display device. Also, exposing of the die bond layer to the high temperature causes weakening of its bonding strength.
In view of the foregoing, a heat-resistant inorganic paste was proposed for the die bond layer, as disclosed in Japanese Patent Application Laying-Open Publication No. 55847/1986. The proposed inorganic die bond paste contains 80-85% by weight of low-melting frit glass mainly consisting of powdered Ag and PbO and an organic binder. Use of the low-melting frit glass for the fluorescent display device exhibits an advantage. More particularly, the low-melting frit glass exhibits fluidity to fully wet the IC chip and anode substrate when it is heated to a sealing temperature of 400.degree.-450.degree. C., as is supposed from the fact that it is used as a sealing material; whereas when it is cooled, it is vitrificated to cause the die bond layer to be in a bulky state, so that it may carry out firm bonding between the IC chip and the anode substrate. Also, when it is heated to a temperature of 300.degree. C. or more, the organic binder contained therein substantially vanishes by combustion and/or volatilization. Accordingly, the above-noted problem encountered with the organic paste is substantially solved.
In general, the anode substrate 4 is made of glass which has a thermal expansion coefficient of about 90.times.10.sup.-7 /.degree.C., whereas a base or substrate of the IC chip 2 is formed of silicon which has a thermal expansion coefficient of about 42.times.10.sup.-7 /.degree.C. Thus, it will be noted that there is a significant difference in the coefficient between both. This causes surfaces of the anode substrate 4 and IC chip 2 bonded together to crack by shrinkage stress due to curing of the die bond layer as both are cooled after firm bonding therebetween. The larger a size of the chip is, the more cracking is.
The fluorescent display device is gradually complicated in its display pattern with advance of its miniaturization, to thereby lead to an increase in the number of anode segments in the display section 1. This causes the number of lead wires and wirings for driving the anode segments of the display section 1 to be concurrently increased. The wirings 7a are conventionally led out directly to an exterior of the envelope, so that arrangement of them on the substrate in the envelope may be relatively readily carried out.
However, in the chip-in-glass type fluorescent display device it is required to connect the wirings 7a to the IC chip 2 in the envelope and connect the IC chip and lead wires together by means of the wirings, resulting in the wirings being required to be arranged at high density on the substrate.
Also, the circuit section 3 of the conventional chip-in-glass type fluorescent display device must be formed while considering the above-described overflowing of the pasty die bond layer 6 from between the IC chip 2 and the die bonding pad 5. This renders arrangement of the wirings 7a between the die bonding pad 5 and the bonding pad 7 substantially impossible, so that density of arrangement of the wirings on a portion of the substrate 4 adjacent to the IC chip 2 is further increased, therefore, manufacturing of the fluorescent display device is highly difficult and troublesome.